The molded article obtained by molding expanded polypropylene resin beads in a mold is excellent in resistance to chemicals, impact resistance, recovery properties of compression stress and the like, for which they have been hitherto widely employed as a shock-absorbing material such as a cushioning material or a core material for bumper.
Conventionally, ordinarily employed polypropylene resins are usually ones, which are obtained by polymerization by use of the Ziegler-Natta catalyst. In recent years, it has been found that those resins prepared by use of a metallocene polymerization catalyst have a melting point lower than that of the polypropylene resin polymerized by use of the Ziegler-Natta catalyst, so that the resultant expanded beads can be molded at a lower steam pressure, and a cell diameter can be made uniform. Thus, there has been proposed the manufacture of a molded article from the expanded beads which make use, as a base resin, of the polypropylene resin polymerized by use of a metallocene polymerization catalyst (Japanese Patent Application Laid-open No. Hei 6-240041).
The present applicant has paid attention to the characteristic properties of the polypropylene resin polymerized by use of a metallocene polymerization catalyst as mentioned above. As a result, it has been found that when homopolypropylenes, which have a problem on moldability, are obtained by polymerization with use of a metallocene polymerization catalyst and, among them, a specific type of homopolypropylene exhibiting specified physical properties is used as a base resin, a molded article can be produced with good moldability by subjecting expanded beads of the homopolypropylene to molding. In addition, the thus produced molded article is more excellent in rigidity, in terms of a compression strain or the like, than prior art counterparts, with an energy absorption efficiency being fully satisfactory. An earlier application based on this finding was filed (as Japanese Patent Application No. Hei 8-229289).
However, a homopolypropylene resin is generally poor in impact resistance at low temperatures, and a resin obtained by copolymerization with a small amount of ethylene or an .alpha.-olefin having four or more carbon atoms should be preferably used as the polypropylene resin. Commercially available homopolypropylene resins, which are obtained by polymerization with use of a metallocene polymerization catalyst, have, at most, a melting point of approximately 150.degree. C., which is lower by 10.degree. C. or over than that of an ordinary homopolypropylene resin polymerized by use of the Ziegler-Natta catalyst. In addition, a copolymer, which is prepared by random copolymerization of propylene with a small amount of ethylene or the like by use of a metallocene polymerization catalyst, lowers the rate of melting point as an amount of ethylene increases with the attendant problem that the molded article obtained from the resultant expanded beads significantly lowers in heat resistance.
Under these circumstances in the art, we have made intensive studies. As a result, it has been unexpectedly found that a molded article obtained from expanded beads using, as a base resin, a copolymer satisfying specified conditions and selected among random copolymers, which are obtained by using, as other comonomer component, one, or two or more in admixture of the comonomers selected from the group consisting of ethylene and .alpha.-olefins having four or more carbon atoms, and copolymerizing them with propylene by use of a metallocene polymerization catalyst, exhibits a higher heat resistance than a molded article of expanded beads composed of a propylene random copolymer, which exhibits substantially the same melting point as the first-mentioned copolymer, is polymerized by use of a metallocene polymerization catalyst and has a high melt flow rate. The invention has been accomplished based on the above finding.
The invention has its object for providing expanded beads of a polypropylene resin, which have good moldability.
The invention has another object for providing a molded article obtained by molding such expanded beads as mentioned above in a mold.
The invention has as its further object for providing a molded article, which has an excellent heat resistance in spite of the fact that the molded article makes use, as a base resin, of a copolymer obtained by random copolymerization of ethylene or the like with propylene by use of a metallocene polymerization catalyst.